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Abstract
In this paper, a 2D optical wireless broadcasting system with low path loss is proposed and experimentally demonstrated based on a commercial liquid crystal on silicon (LCoS) device and rotated-splitting-SLM algorithm. The low path loss is realized by our designed pair of fiber collimators with large beam waist, which can significantly reduce the coupling loss. By using the proposed system and algorithm, a 60-Gb/s PAM-4 signal is successfully broadcasted over 2-km standard single mode fiber and 14 m free space link with 16 dB path loss for up to four mobile users. The experimental results reveal that the LCoS based on the proposed rotated-splitting-SLM method can be used for multi-user broadcasting in the high-speed in-building dynamic networks.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Recently, the exponential growth of personal mobile devices connected to the wireless networks makes the current wireless transmission traffic more vulnerable [1–3]. In the upcoming 5G wireless networks, the applications in mobile phone and computers such as online games, virtual reality (VR) and augmented reality (AR) require high access data rates and low delay service [4,5]. Optical wireless communication (OWC) using infrared wavelengths have attracted much attention as it can provide almost unlimited bandwidth to achieve the ultra-high capacity. In a beam steered OWC system, an active device like micro electro mechanical systems (MEMS) mirror [6,7] or liquid crystal on silicon (LCoS) [8,9] is usually needed to steer the optical beams to the users located in an indoor area. The use of MEMS mirror will not introduce obvious additional distortion on the optical beams such as beam aberration and optical power loss. But it is hard to realize point to multi-point transmission by using the MEMS mirrors since a moving part is required to mechanically steer the beam.
Two different approaches to realize point to multi-point service in the indoor wireless distribution system have been reported in [10]. The first one is using a passive dispersive element to separate the different wavelengths into different angles. Then the optical carrier with different wavelengths carrying the modulated optical signal is transmitted through the free space to the specific user, which means each user has its own exclusive wavelength. Grating based multi-users OWC systems have been presented in [11,12]. Multiple infrared beams with different wavelengths for multi-users are realized by using a broadband light source at the transmitter side. Array waveguide gratings router (AWGR) can also be used to realize multi-users service in OWC systems as demonstrated in [13,14]. Wavelength-controlled two-dimensional (2D) beam steering is achieved by means of a high port-count AWGR and a fiber array. The output fibers are arranged in a 2D array placed at the front of an optical lens to form a 2D angular direction. The other one is using a LCoS to generate multiple beams in single wavelength, and each beam carries the same optical signal for different users. This architecture enables to increase the number of served users and the transmission capacity of the OWC systems with low system complexity. In [10], point to two-point OWC system was demonstrated by using a single spatial light modulator (SLM) with the technology of splitting-SLM and Gerchberg-Saxton (GS) algorithms. The experimental results show that the performance of the splitting-SLM method outperforms that of GS method with less hologram generation time. However, the two beams are limited to one-dimensional (1D) and the situation of more than two generated beams is not discussed and demonstrated. In [15], a holographic beam steering based optical wireless upstream and downstream data transmission system was presented for two nomadic users. But this architecture introduces almost 20 dB power loss over 4 m free space transmission link. Moreover, the high complexity of the holographic image generation increases the configurable time of the system, which hinders the feasibility of practical application.
In this paper, we experimentally demonstrate a high-speed OWC system based on a LCoS device supporting 2D broadcasting service for multiple users, which is realized by the rotated-splitting-SLM algorithm. We also design and fabricate a pair of fiber collimators with large beam waist to reduce the coupling loss. By using the proposed system and rotated-splitting-SLM techniques, a reconfigurable 2D broadcasting link over 2-km standard single-mode fiber (SSMF) and 14 m free space is successfully achieved with up to 60-Gb/s PAM-4 data rate. The measured results provide good performance for supporting simultaneously four users with BER values of less than 10−5.
2. Operation principle
Figure 1 depicts the schematic diagram of the proposed 2D optical wireless broadcasting system based on a LCoS-SLM device and rotated-splitting-algorithm. The high-speed fiber distribution access network equipped with the beam steering and broadcasting (BSB) modules is connected by a communication control center (CCC) via an optical cross connector (OXC). The BSB module on the ceiling of the building can send the optical signal to the mobile devices (MDs) on the ground in the room area. At the free space transmitter, the optical power is limited to 10dBm for eye safety requirement. Thus, the appropriate way to achieve long-distance and high-speed data transmission is to reduce the coupling loss during the transmission, since the received signal performance is proportional to the receiver optical power and the indoor path loss is negligible. In our proposed scheme, a pair of matched fiber collimators is employed at the BSB module and MD sides, which can significantly improve the value of optical power coupling into the receiver fiber. The LCoS-SLM at the BSB module enables to steer and split the input beam into different output beams with different angles of emergence. The output beams then travel in the free space to the MD side. The MDs receive the optical signal by the matched collimators.
2.1 Optical design to realize low coupling loss
In the traditional OWC systems, the coupling loss between the free space and fiber takes the huge part of the optical power loss in the transmission system. A pair of fiber collimators are usually utilized at the transmitter and receiver sides for an OWC system. The normalized coupling loss in the presence of separation misalignment between the transmitter and receiver collimators is express as [16,17]:
where λ is the wavelength of incident beam, ω is the Gaussian beam waist (radius), and Z is the separation distance between the two collimators.The divergence angle θ of the output Gaussian beam is then described by:
According to Eqs. (1) and (2), when the wavelength λ is fixed, the tolerance distance and the divergence angle is only related to the Gaussian beam waist. As increasing the Gaussian beam waist, the divergence of the beam decreases, which helps to enlarge the tolerance distance. That means the output Gaussian beam with larger beam waist is easier to be coupled. Figure 2 shows the coupling loss for a Gaussian beam at λ=1550 nm as a function of separation distance with Gaussian waist of 1 mm and 6.5 mm. When the separation distance Z increases to 14 m, the coupling losses are about 25.6 dB and 0.068 dB for ω=1 mm and 6.5 mm, respectively. Thus, the coupling loss is very sensitive to the Gaussian beam waist. Fortunately, when the Gaussian beam waist is designed to 6.5 mm, the coupling loss can be ignored even after 14 m transmission as shown in Fig. 2. We design and fabricate a pair of fiber collimator to generate a Gaussian beam with the waist of 6.5 mm and implement them in the experiment, which is suitable for long distance OWC systems.
Fig. 2. Coupling loss versus separation distance with different beam waist, top: ω=1 mm, bottom: ω=6.5 mm.
2.2 2D broadcasting
To realize the broadcasting function also called point to multi-point function by using a LCoS-SLM, there are two common methods by using splitting-SLM algorithm and GS algorithm [10]. Here, we choose splitting-SLM method to achieve broadcasting function for two reasons. First, the performance of splitting-SLM method outperforms that of the GS method by experiment as the hologram image can be simply and fast generated. Second, when LCoS is used for beam steering, a large number of pixels on the steering axis need to be addressed to realize a stair-stepping phase distribution especially for multi-users. Since the fiber collimators with large beam waist are designed and used in our experiment, splitting-SLM algorithm is preferred. To realize the capability of serving N-users simultaneously, we need to divide the LCoS active area into N independent programmable sections. Then, different stair-stepping phased distributions are applied on these independent sections. Thus, the single input large optical beam illuminating onto the LCoS chip is split into N separated beams in the free space with respective output angles. Here, we further propose a rotated-splitting-SLM method to extend the 1D optical broadcasting to 2D.
By rotating the whole hologram image by an angle of β, the directions of the corresponding output beams can be deflected by the angle of β at the receiver plane [18], which forms the 2D broadcasting. The value of β belongs to [0, Ф], and Ф is defined as the spacing angle of the separated beams on the distributing circle, which can be expressed as:
where N is the number of the expected output beams. Taking N=4 for example, Ф=90°. That means when rotating the whole hologram image by 90°, the output beams have a cycle rotation. At the receiver plane, the four beams are distributed on a circle with radius of R, where R = L*tanα, L is the distance between the LCoS chip and the receiver plane. Figure 3 shows the principle of LCoS based 2D optical broadcasting for four reconfigurable output beams by using the rotated-splitting-SLM algorithm. By changing the grating pitch of the four independent active sections as shown in Figs. 3(a) and 3(b), the steering angle α can be realized from 1° to 3° for the four different output beams [8], which forms the 1D beam steering. By using an angle magnification configuration proposed in [19], the field of view can be enlarged to 30°. By further rotating the hologram image by 45° as shown in Figs. 3(a) and 3(c), the steering angle at the receiver plane is changed from 0° to 45° to generate a 2D broadcasting.Fig. 3. Principle of the rotated-splitting-SLM method that supports 2D continuous broadcasting: (a) α=1°, β=0°; (b) α=3°, β = 0°; (c) α=1°, β=45°; and (d) α=3°, β=45°. α is the steered angle at the incident plane, β is the rotated angle of the hologram image, L is the distance between the LCoS chip and the receiver plane, R is the radius of the beam distributing circle at receiver plane, and Φ is the spacing angle of the separated beams on the distributing circle.
3. Experiment for broadcasting
3.1 Spot distribution detection
We design a preliminary experiment to evaluate the broadcasting function. In the experiment, the used LCoS-SLM is produced by HOLOEYE Company with 1080×1920 resolution. The pixel pitch is 8µm and the fill fact is 93%. As shown in Fig. 4, an optical beam with 13 mm diameter (whose beam waist is 6.5 mm) from the transmitter collimator launches onto the LCoS chip surface. The LCoS is downloaded with different phase distribution images to divide the input beam into several discrete ones. Due to the large size of the output beams, we use a focusing lens to focus the discrete optical beams to be collected by the CCD more easily as shown in Fig. 4. The used focusing lens in the experiment has the focal length of 100 mm. The distance between the focusing lens and the LCoS chip is set to be more than the focal length and the distance between focusing lens and CCD is less than the focal length.
We further generate the hologram images by a computer and download them onto the LCoS as shown in Figs. 5(a) to 5(f). The corresponding multiple optical beams are distributed in free space and detected by the CCD (OPHIR Photonics, BM-USB-SP907-1550-OSI). When the stair-stepping phase distribution is applied as illustrated in Fig. 5(a), the LCoS loaded with the linear phase function acts like a paced mirror with calculated reflect angle, and the steering function is realized. The edge of the optical beam is cut by the LCoS chip, so the detected beam shape is not a perfect circular as shown in Fig. 5(g). Using the splitting-SLM method, phase distributions generated by computer hologram techniques for N=2, 3, 4, 8 are successfully achieved as shown in Figs. 5(b) to 5(e). The corresponding beam intensity distributions with equal powers are measured and shown in Figs. 5(h) to 5(k), respectively. The hologram image shown in Fig. 5(f) is rotated by an angle of 45° compared with the hologram image shown in Fig. 5(d), whose beam spot distribution is also rotated by 45° as shown in Fig. 5(l). It is noted that the spot size of each discrete optical beam is reduced when the number of the separated optical beams increases. The transmitted optical power is limited for each discrete optical beam under eye safety requirement. Smaller beam size means the more focusing of the energy and this is useful for long distance transmission.
Fig. 5. (a) ∼ (f) Generated different hologram images, and (g) ∼ (l) measured beam spot distributions by a CCD.
3.2 Data transmission link design
Figure 6 shows the experiment of the proposed 2D optical wireless broadcasting system modulated by 60-Gb/s PAM-4 signal by using a LCoS-SLM. The used optical source is an external cavity laser (ECL) from Keysight and its operation wavelength is set at 1550 nm, which is then modulated by a Maher-Zender intensity modulator. An arbitrary waveform generator (AWG) (Keysight, M9502A) running at 30-GSa/s is used to produce the 60-Gb/s electrical PAM-4 signals with bit length of 216. The peak-to-peak voltage (Vpp) of the electrical PAM-4 is amplified to 1.2 V by an electrical amplifier and the direct current voltage (VDC) is set to 4.5 V to drive the modulator. The 60-Gb/s optical PAM-4 signals are amplified to 10dBm by an Erbium Doped Optical Fiber Amplifier (EDFA-1) after transmission over 2 km SSMF, whose function is to emulate the in-building fiber distribution network. Since the LCoS is a polarization sensitive device, a polarization controller (PC) is used to make sure the lowest polarization dependent loss before the optical signal arrives at the LCoS chip surface. The modulated optical beam is then fed into the free space by the designed fiber collimator. In the experiment, a pair of matched fiber collimators with the same optical parameters are designed to reduce the coupling loss and used to extend the optical wireless transmission distance. The operation wavelength of the collimator is 1550 nm, and its beam waist is 6.5 mm. The divergence angle θ of the collimator is calculated to be less than 0.1mrad. After transmission over 3.5 m free space distance, the optical beam arrives at the LCoS surface. The optical beam is tailored by the LCoS to a desired angle in a single beam or separated into multiple beams. Two reflector mirrors are employed to extend the distance by three times. Thus, the generated optical beams with different output angles are transmitted in free space for total 14 m to the user side. The matched fiber collimator sat the receiver are used to couple the multiple free space optical beams into SSMF, respectively. The received optical signal is first amplified by another EDFA (EDFA-2) and then detected by a PIN photodiode (PD) (Discovery, DSC10ER). It is noticed that the EDFA-2 is used to improve the receiver signal-to-noise-ratio, since no electrical amplifier is employed after the PD. The electrical PAM-4 signal is over sampled at 80 GSa/s by a digital storage oscilloscope (DSO) (TeledyneLeCroy, 10-59ZI-A) for further offline signal demodulation and demapping after feed forward channel equalization. The recovered bits after demapping is compared with the original transmitted bits for BER calculation. After demodulation and demapping the PAM-4 data, the received bit stream is achieved, which is compared with the transmitted bits. Thus, the number of the errors are obtained experimentally. The value of BER is finally calculated by the number of errors divided by the length of the transmitted bits.
Fig. 6. (a) Experimental setup of the 2D optical wireless broadcasting system modulated by 60-Gb/s PAM-4 signal by using a LCoS-SLM and (b) The experimental site (the MZM modulator, 2 km SSMF and EDFA-1 are not included in the picture). AWG: arbitrary waveform generator; MZM: Maher-Zender modulator; SSMF: standard single mode fiber; EDFA: Erbium doped fiber amplifier; PC: polarization controller; PD: photodiode; DSO: digital storage oscilloscope.
4. Results and discussions
4.1 Path loss measurement
We first measure the total optical power loss after 7 m and 14 m free space transmission at different broadcasting schemes as shown in Fig. 7. The power losses at back-to-back (BTB) between the transmitter and receiver collimators after 7 m and 14 m free space link are 8.5 dB and 10.6 dB in our measurement. Here, the BTB is defined as no phased modulation applied onto the LCoS, which means that the LCoS just acts as a reflect mirror. Such power loss is mainly induced by two parts. The first one is the absorption and diffraction loss introduced by the gap between the pixels, since the fill fact of the LCoS is not 100% [20,21]. The other one is the cutting loss introduced by the edge of LCoS. When the LCoS is used for beam steering, two different steering angles of 3° and −3° are confirmed. Limited by the number of receiver collimators, we move the focusing lens and PD at the right location for every beam to achieve the best performance. When the steering angle is 3°, the power losses of the transmission system are 10.7 dB and 12.8 dB at 7 m and 14 m distance, respectively. When the steering angle is −3°, the power losses are measured to be 10.8 dB and 12.7 dB after 7 m and 14 m transmission, respectively. Compared with the BTB case, the increased power loss is introduced by the diffraction loss caused by the unwanted high order diffraction when the stair-stepping phased modulation is applied onto the LCoS [20,22], and the polarization dependent loss. We also measure the power losses for the broadcasted multiple discrete beams as exhibited in Fig. 7. It is worth noting that the splitting loss is not included in calculation at the broadcasting case. The power loss gets worse as increasing the number of the beams divided by LCoS. Compared with the reported works in [18], the total optical power loss is about 28 dB over 0.5 m free space without an angle magnification configuration. Here, we successfully achieve the transmission distance of 14 m with the total power loss of 16 dB for a 2D broadcasting with four users. If the angle magnification configuration with aberration correction algorithm as demonstrated in [19] is used in our four-user broadcasting system, the total power loss for each user will increase to 30 dB.
4.2 Link performance evaluation
Figure 8 shows the BER performances of 60-Gb/s PAM-4 signal as a function of received optical power (ROP) for the different broadcasting cases. The launched optical powers to the free space at the transmitter for both BTB and beam steering cases are 10dBm. Theoretically, the splitting losses for two users, three users, and four users are 3 dB, 4.8 dB and 6 dB, respectively. Therefore, considering the eye safety requirement, the transmitted optical powers from the fiber collimator are set to 13dBm and 14.8dBm and 16dBm for broadcasting two, three and four users, respectively. Here the EDFA-2 provided 10 dB gain to the optical signal detected by the receiver collimator. Compared with the BTB performance, about 0.2 dB power penalty is observed for the beam steering case at the BER level of 1×10−4 as shown in Fig. 8(a). While about 0.4 dB, 0.6 dB and 0.7 dB power penalties at the BER level of 1×10−4 are measured for broadcasting two, three, and four users as shown in Figs. 8(b), 8(c) and 8(d), respectively. Such penalties are mainly caused by the different optical power losses for broadcasting multiple users, which can be verified by the measured results as shown in Fig. 7. The broadcasted each user has the same BER performance which below the enhanced FEC limit (BER level of 3.8×10−3) have been achieved as shown in Fig. 8, which reveals the feasibility of our proposed 2D optical wireless broadcasting system. The recovered eye diagrams for the 60-Gb/s PAM-4 signals for the broadcasting cases at the BER values of 10−5 (Top), 10−4 (Middle) and 10−3 (Bottom) are also presented in Fig. 9.
Fig. 8. BER performance of 60-Gb/s PAM-4 signal versus receive optical power for different broadcasting schemes. (a) Beam steering for 2 users, (b) beam broadcasting for 2 users, (c) beam broadcasting for 3 users, and (d) beam broadcasting for 4 users.
Fig. 9. Measured eye diagrams for broadcasting for four users with different BER level. (a) Broadcasting for 4 users #1, ROP=3.6 dBm; (b) Broadcasting for 4 users #2, ROP=3.8 dBm; (c) Rotated broadcasting for 4 users #1, ROP=4 dBm; (d) Rotated broadcasting for 4 users #2, ROP=3.9 dBm; (e) Broadcasting for 4 users #1, ROP=2.6 dBm; (f) Broadcasting for 4 users #2, ROP=2.8 dBm; (g) Rotated broadcasting for 4 users #1, ROP=3 dBm; (h) Rotated broadcasting for 4 users #2, ROP=2.9 dBm, (i) Broadcasting for 4 users #1, ROP=1.6 dBm; (j) Broadcasting for 4 users #2, ROP=1.8 dBm; (k) Rotated broadcasting for 4 users #1, ROP=2 dBm; (l) Rotated broadcasting for 4 users #2, ROP=1.9 dBm.
5. Conclusions
We have proposed and experimentally demonstrated a 2D optical wireless broadcasting system enabled by a commercial LCoS device and rotated-splitting-SLM algorithm. A pair of matched fiber collimators are designed and used in the experiment to reduce the coupling loss and achieve low power penalty. Based on the above techniques, a 60-Gb/s optical PAM-4 signal has been successfully transmitted over 2-km SSMF and 14 m free space link serving for four users. The experimental results show that the high-speed reconfigurable broadcasting system for the in-building dynamic networks can be realized by a single LCoS device.
Funding
National Key Research and Development Program of China (No.2019YFB2203203).
Acknowledgments
The authors thank Dr. Chao Yang, Dr. Wu Liu and Mr. Zichen Liu from China Information Communication Technologies Group Corporation for the fruitful discussion on the experiment setup and result analysis.
Disclosures
The authors declare no conflicts of interest.
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